Electronic oven



May 16, 1967 H. BOEHM 3,320,396

ELECTRONIC OVEN Filed June 18, 1964 2 Sheets-Sheet 1 ,-1 HEAT CONTRCLLER. 1$ g, SELECTOR H 28 i l6 V F] 20 l 60 w, e as/6o Z? 61 MM. 32 PovuER SUPPLY I v ELMT ck 22 md: B hm iwgt mwmwcaw May 16, 1957 H. BOEHM 3,320,396

ELECTRONIC OVEN Filed June 18, 1964 2 Sheets-Sheet 2 VII/VII)? SUPPLY 63 i t L2 I l: l W Q-(elmdl'i GO h L,

mz m wzmm United States Patent 3,320,396 ELECTRUNHC OVEN Helmut Boehm, Rockford, Ill., assignor, by Inesne assignments, to Technology Instrument Corporation, Newhury Park, Califl, a corporation of California Filed June 18, 1964, Ser. No. 376,185 17 Claims. (Cl. 219-1il.55)

The present invention relates in general to ovens for heating objects placed therein, and in particular to ovens of the type which employ microwave energy for locally and internally heating objects containing appreciable amounts of moisture. While not so limited in its uses, the invention will find especially advantageous employment in connection with ovens utilizing microwave energy, together with more conventional low frequency heat energy, for cooking foods in homes, restaurants, and selfservice cafeterias.

The general aim of the invent-ion is to provide an oven for heating objects placed therein either by microwave energy or by more conventional lower frequency heat energy, or by both, and to accomplish this by the action of common elements which perform important functions in both types of heating.

It is a related object to achieve the foregoing while at the same time reducing the number and cost of components, and the space occupied by them, in an oven chamber.

More specifically, an important object of the invention is to provide an oven in which a single element serves both as an antenna for radiating microwave energy in the oven chamber and as a resistor type heater for creating heat within the oven chamber.

A related object is to provide such an oven in which a single element operates to heat the food or other object therein either by microwave energy action alone, by resistance heating action (1 R dissipation) alone, or simultaneously by both such actions.

It is a further object to provide an oven employing a source of microwave energy together with a high voltage power supply therefor which employs a voltage regulating resistor, and to avoid the need to remove (for example, by a special cooling fan) substantial amounts of heat created in the resistor while at the same time effectively using such heat to good advantage in the oven chamber itself.

Another object is to achieve the foregoing by the use of a resistor which need not be especially and expensively constructed for high voltage operation, without the need for dangerous high voltage potentials and special insulators in the oven chamber, and in a manner which makes it possible to employ a smaller transformer in the high voltage supply circuit.

Still another object is to provide such an oven arrangement in which an element employed as a microwave radiating antenna in the oven chamber serves also as a voltage regulating resistor in the high voltage supply for the microwave source, that element thus contributing to the heating of an object in the oven by transferring microwave energy to the object and simultaneously radiating conventional lower frequency heat created by current flow through the element as an incident to its voltage regulating operation.

It is a related object to provide such an oven in which a voltage regulating resistor connected in the circuit of a high voltage supply for a microwave energy source is disposed in the oven chamber, and yet in which that resistor may be selectively connected to a conventional voltage source so as to heat the oven by direct action when the microwave source is not being used.

Stated another way, it is another object here to provide an oven in which a single element within the oven chamber 3,320,395 Patented May 16, 1967 acts alternatively or simultaneously (a) to radiate microwave energy, (b) to perform an important voltage regulating function in a high voltage supply for a microwave generator, such as a magnetron, and (c) to generate heat by the action of current flowing through a resistance, the latter occurring either as an incident to operation of the high voltage supply circuit or as the result of current drawn from conventional A.C. power means.

Other objects and advantages will :become apparent as the following description proceeds, taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic illustration of an oven constituting an exemplary embodiment of the present invention, and showing in vertical section an oven chamber together with the heating means and controls therefor;

FIG. 2 is a perspective view showing the configuration of multi-purpose rod-like elements disposed in the oven chamber of FIG. 1;

FIGS. 3 and 4 are fragmentary sectional view respectively showing the construction details of the upper and lower rod-like elements;

FIG. 5 is a schematic diagram of a high voltage supply which creates and applies an operating voltage to a microwave energy source, e.g., a magnetron; and

FIG. 6 is a schematic circuit diagram, including the high voltage supply shown in block form, for the selector and control means associated with the oven.

While the invention has been shown and will be described in some detail with reference to a particular embodiment thereof, there is no intention that it thus be limited to such detail. On the contrary, it is intended here to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the appended claims.

Referring now to FIG. 1, the oven here shown by way of example to illustrate one embodiment of the invention includes structural means for defining an oven chamber 10 which is adapted to receive and surround an object 11 to be heated. The chamber is formed by six walls disposed in box-like configuration, these including upper and lower walls 12 and 14, a rear wall 15, two side walls (not visible), and a front wall constituted by a door 16 provided with a handle 17 and pivoted at 18 to swing between open and closed positions. Each of these walls is preferably formed by spaced apart sheet metal panels, with insulating materials disposed between them. Preferably small openings are provided to permit limited circulation of air and the escape of water vapors from the chamber 10, holes being illustrated at 19 and sized so that they do not permit escape of appreciable amounts of microwave energy from the chamber.

The object to be heated may be, for example, an item of food containing significant amounts of moisture. -It is disposed in a non-metallic (e.g., paper, plastic, or glass) utensil 20 inserted through the door 16 and placed for support on an open framework metal shelf 21 which is formed to permit passage of microwave and heat energy therethrough.

In accordance with the present invention, a metal element is disposed within the chamber 10 and formed in the shape of an antenna for radiating microwave energy. That element includes, however, as an integral part thereof a resistive conductor adapted to have current at a frequency much lower than microwave frequencies \(e.g. direct current or conventional 60 cycle A.C. current) passed therethrough and thus to dissipate heat energy according to the known relationship of current squared times resistance (1 R) which determines the rate of heat generation in watts. Heating by current flow through a resistance will hereinafter be termed resistance heating or PR dissiption, the heat energy so created within the oven chamber being at much lower frequencies (longer wave lengths) than energy in the higher frequency, micro-wavelength range.

Two such elements 24 and 25 are employed in the present instance. They are preferably constituted as rodlike members folded back upon themselves to lie in upper and lower horizontal planes (FIGS. 1 and 2). Provision is made, as hereinafter described, to couple high frequency microwave electrical energy to the elements 24, 25 so that they act as antennas which radiate .that energy into the oven chamber where it is absorbed by and causes heating within the food item or object 11. Additionally, however, means are provided to pass current through the resistive conductors of the elements 24, 25 so that the latter also contribute heat within the chamber 10 by a resistance heating mode of operation. This latter heat energy serves to heat the food item or object 11, and indeed to produce a browning action on the surface thereof which is not normally produced by the action of microwave energy.

As will become apparent from the more detailed description which follows, the elements 24, 25 may function solely to radiate microwave energy, solely to produce resistance type heating action, or simultaneously to produce both microwave and resistance type heating actions.

As made clear by FIG. 1, the elements 24 and 25 are appropriately supported (by means not shown) within the chamber 10, one extremity 24a, 25a of each passing in non-conducting relation through openings 26, 27 in a metal partition 28 and thence through elongated, metal walled passages 29, 30 formed in the rear wall to terminal blocks 31 and 32. The opposite extremities 24b, 25b of these elements pass through the rear wall 15 to the respective terminal blocks 31 and 32, but are in electrical contact at points 34, 35 with the inner metal panel of that wall for reason to be more fully explained below.

Although the specific structure of the elements 24 and 25 may vary, and indeed both the radiating antenna function and the resistance heating function may be accomplished by a single metal piece, preferred forms thereof are shown by FIGS. 3 and 4, respectively. As shown in FIG. 4, the lower element 25 is constituted by an outer metal sheath 38 which surrounds and is spaced or insulated from an inner resistive conductor 39 made of any suitable high resistivity metal such as nickelchromium alloys, molybdenum, or other resistance heater materials well known in the art. The inner resistive conductor 39, although shown in the form of simply a straight wire, constitutes a heater type resistor which generates substantial low frequency heat and energy in response to the passage of energizing current therethrough. The space between the sheath 38 and the resistive conductor 39 may be filled with a heat-resistant, inert insulating powder 40 so that the rod-like element after initial manufacture can be bent or otherwise formed into a folded-back or serpentine configuration such as that illustrated in FIG. 2.

One product available on the market and which may readily be employed as the element 25 is known by the trademark Calrod. Such Calrod units will serve satisfactorily, but it is preferred that the outer sheath 38 thereof be made of a low resistivity metal which tolerates high temperatures, so as to present low resistance to current which may flow therethrough at microwave frequencies as an incident to radiation of microwave energy.

In or near the junction block 32 the tips of the element 25 are terminated by an insulating disc 41 pressed into the sheath 38 and retained in place by a nut 42 threaded onto the sheath. The resistive conductor or resistor wire 39 extends through the insulating disc 41 and passes with clearance through an oversize opening 42a in the nut so as to remain insulated from the sheath. The projecting portion of the resistor wire 39 then extends into connector sockets carried by the terminal block 32 and electrically connected with terminal screws shown on the upper surface of the block.

The rod-like element 24 is here shown as being identical in every way with the element 25, except that the former contains two separate and spaced-apart inner resistive conductors 44 and 45. Accordingly, the individual components of the element 24 shown in FIG. 3 are identified by the same reference characters employed in FIG. 4, except for the distinguishing addition of prime symbols. Whereas the opposite ends of the resistor wire 39 lead to terminals 39a, 3% on the terminal block 32 (FIG. 2), the terminal block 31 has four connection points, its first two terminals 44a, 44b being connected to the opposite ends of the resistor wire 44 and its second two terminals 45a, 45b connecting with the opposite ends of the resistor wire 45. p

In keeping with the invention, the resistive conductors 44 and 45 are adapted for connection to a conventional voltage source so that they may create heat by PR dissipation within the chamber 10. Such connection is established, for example, through a heat controller and selector 50 (FIG. 1) to conventional 220 Volt AC. power mains L1, L2 and controlled in response to oven temperature as sensed by a thermostatic unit 51 disposed in the chamber 10.

The controller and selector 50 is shown in somewhat more detail by FIG. 6. A main selector switch 52, closed when resistance heating action is desired, leads from the lines L1, L2 through thermostatic switch contacts 51a to the input terminals 55a, 55b of a selector switch S having ganged switch arms S1, S2, S3 set to any one of four associated contact points a, b, c, d, by manual adjustment of a selector knob Sk. Assuming that the main switch 52 is closed, and the oven temperature is below a desired value so that the thermostatically controlled switch 51a is also closed, then neither the heater resistor 45 nor the heater resistor 44 will be energized when the selector knob Sk is in the off position. If that knob is turned to the low position, a circuit is established from line L1 through contacts S1, Slb, the resistor 45, the resistor 44, and contacts S3b, S3 to line L2. In this way, the two resistive conductors 44, 45 are connected in series across the 220 volt power source. If, on the other hand, the knob Sk is set to the medium position, a circuit will be established from line L1 through contacts S1, S10, resistor 45, and contacts S3b, S3 to line L2. This applies 220 volts to the heater resistor 45, but leaves the second heater resistor 44 in the element 24 de-energized. Finally, if the knob Sk is set in the high position, a circuit is established from the line L1 through the contacts S1, Sld, through the resistor 45, and thence through the contacts S3d, S3 to the line L2; while a similar circuit is established from the line L1 through the contacts S2, S2d through the resistor 44 and the contacts S3d, S3 to the line L2. Thus, the two resistors 44 and 45 are connected in parallel across the 220 yolt power source so both receive relatively large energizmg currents and generate heat at a high rate.

The selector switch S, therefore, permits the conductive resistances 44 and 45 of the element 24, to produce either a low, medium or high rate of heating in the chamber 10 by resistance heating action. This heating of the chamber 10 may, if desired, be controlled by the thermostatic unit 51 which operates to open or close the switch 51a as the temperature within the chamber rises above or falls below some predetermined and selected value. It will be apparent from FIG. 6 that as the contacts 51a open or close, the energizing circuit or the selected combination of the heater resistors 44 and 45 is broken or completed. In keeping with another aspect of the invention, however, means are also provided to couple high frequency microwave electric energy to the elements 24 and 25 so that they serve the additional function of radiating microwave energy throughout the chamber and toward the object 11 to be heated To accomplish this, a suitable source of microwave energy has its output coupled by high frequency energy-transmitting means to the rod-like elements 2425. While the microwave energy source may take various forms such as high frequency triode oscillators or klystron tubes, that here shown in FIG. 1 is a magnetron 60 schematically illustrated as comprising a multi-cavity anode 60a surrounding a cathode 60b. The magnet and the cathode heater normally associated with such a magnetron are not illustrated, inasmuch as there components are well known to those skilled in the art of magnetrons and microwave heating. The operating energy for the magnetron 60 is supplied to it in the form of a relatively high direct voltage applied between its anode and cathode, and for this purpose a high voltage power supply 61 is here shown as having its output terminals 61a and 61b connected respectively to the anode 60a and the cathode 60b. The input terminals 61c and 61d are adapted for connection to a conventional voltage source, here represented by the 220 volt AC. power main L1, L2.

It is well known that a magnetron when so supplied with an operating voltage serves as an oscillator which generates very high frequency electrical energy. By appropriate design or adjustment of the magnetron, it can be made to produce microwave energy in the ultra-high frequency range of 2,000 to 3,000 megacycles, the most commonly employed microwave cooking frequency being about 2,450 megacycles.

For the purpose of transmitting the microwave energy output of the magnetron 60 to the elements 24 and 25, a coupler in the form of a probe 600 is disposed in one anode cavity and projects through a coaxial sheath 62 into a metal wave guide 64. Microwave energy is thus transmitted and reflected through the wave guide 64 to a T junction 65 where it divides and passes upwardly and downwardly through wave guides 66 and 67 defined in part by the metal partition 28 and the inner metal panel of the wall 15. For efficienttransfer of energy passing through the wave guides 66 and 67 to the elements 24 and 25, the latter are disposed with a spacing equal to a quarter wave length at the magnetron frequency from the upper and lower extremities of the wave guides.

The metal walled passages 29 and 30 through the wall are closed by annular metal plates 69 and 70 spaced approximately one quarter wave length from the inner panel of the wall, thus minimizing the escape of microwave energy toward the terminal blocks 31 and 32. These quarter wave length shorted stubs appear as very high impedances, and thus cause the microwave energy to be transferred to the elements 24 and 25 which have their opposite ends electrically connected at 34 and 35 to the inner metal panel of the wall 15.

With this arrangement, microwave energy generated by the magnetron 60 passes through the wave guide 64. divides and passes through the upper and lower wave guides 66 and 67, and is then coupled to the elements 24 and 25 so that the metallic sheaths 38' and 38 thereof act as radiating antennas which distribute the microwave energy within the chamber 10. Thus, it may be said in the parlance of electrical engineers that elements 24 and 25 are driven by microwave energy coupled to them from the microwave generator, and they act as driven antennas which radiate such microwave energy into the oven chamber. As is known in the art of microwave heating, this energy will be reflected from the internal metal surfaces which define the chamber 10, will pass without substantial attenuation or loss through nonconductive materials having negligible dielectric losses (such aspaper, glass or certain plastics), and will be absorbed in dielectric materials such as moisture-containing food items 11 disposed in the utensil 20. The microwave energy is effective in heating the moisturecontaining object 11 substantially uniformly both at its outer surface and in its center portion, and it acts with great speed in raising the temperature of the object to a relatively high level. Thus, it is possible to heat the object 11 and quickly to cook it if it is an item of food, simply by the microwave energy generated in the magnetron 60 and radiated by the elements 24 and 25 acting as antennas.

As mentioned above, the microwave generator or magnetron 60 receives a high operating voltage from the voltage supply 61 having its input terminals 61c, 61d adapted for connection to the AC. power source lines L1,, L2. The high voltage supply may be controlled by a second thermostatic sensor 71 disposed in or near the object 11 so as to indicate directly or indirectly the temperature of the latter. The sensor 71 controls a switch which renders the power supply 61 effective or ineffective, thereby turning the microwave generator on or ofl as the sensed temperature falls below or rises above a desired value.

In accordance with another feature of the invention, the power supply 61 is so constructed as to include the element 25, or more properly the resistive conductor 39 forming a part thereof, as one component which performs an important voltage regulating function. As an incident to that function, however, the resistive conductor generates a considerable amount of heat, and the present system not only avoids the necessity of dissipating such heat by a special cooling fan or the like, but also makes advantageous use of such heat by causing it to be distributed within the oven chamber 10 where it contributes to the warming of the object 11.

As shown in greater detail by FIG. 5, the high voltage supply 61 has its input terminals 61c, 61d adapted for connection to the voltage source lines L1, L2 through contacts C1 of a control switch C. The switch C is shown in FIG. 6 in its normal position to which it is set when heating by microwave energy is not desired, and it is shown in FIG. 5 in the actuated position to render the power supply 61 and the magnetron 60 effective. The switch C has normally open contacts C1, normally closed contacts C2, and normally open contacts C3. With these contacts actuated (FIG. 5), the high voltage supply 61 includes an input series circuit from the line L1 through the primary winding Tp of a step-up transformer T, through one side of the contacts C2 to the terminal 391), through the resistor 39 to the terminal 39a, through the other side of the contacts C2, and thence through contacts 71a (controlled by the thermostatic sensor 71) to the line L2 The secondary winding Ts of the transformer T is connected to the input of a full wave rectifier 75, the output of the latter being connected through the terminals 61a, 61b to the anode 60a and cathode 60b with a polarity to make the anode positive relative to the cathode.

With the power supply energized by the switch C set to the position shown in FIG. 5, successive pulses of very high voltage are applied to the anode 60a by virtue of the full wave rectified output of the step-up transformer T. These pulses will have a repetition rate of c.p.s. if the frequency of the AC. source voltage on lines L1, L2 is 60 c.p.s. As each such pulse of anode voltage rises above a certain level, the magnetron is triggered into oscillation, and generates energy at a frequency of about 2,450 megacycles per second until the anode voltage pulse again falls below a cutoff level. In the exemplary arrangement, therefore, the magnetron 60 does not continuously oscillate, but rather produces short bursts of microwave energy, each such burst being at a frequency of 2,450 megacycles, and with the bursts recurring at a repetition rate of 120 c.p.s.

The power or wattage of microwave energy drawn from the magnetron 60 depends upon the quality and quantity of the energy absorptive material (such as the object 11) in the chamber 10, and upon the impedance matching back through the wave guides 66, 67, 64 to the output coupling loop in the magnetron. It is characteristic of magnetrons and other similar microwave generators that the cathodeanode current varies not only with the output power coupled to the load, but also with the anode-cathode voltage. While a relatively high anode voltage is required to initiate high frequency oscillations, once they have been started any further increase in anode voltage causes a severely and disproportionate increase in cathode current. Unless some provision is made to regulate and limit the anode voltage, the current flow may become so high as to damage or destroy the magnetron, for example, by injuring the electron-emissive material on the cathode or overheating of structural parts.

This characteristic of magnetrons is well known, and it has been dealt with in the past by providing a voltage regulator in the form of a feedback-controlled saturable reactor or the like connected in series with the primary winding of the step-up transformer. In the practice of the present invention, however, the cathode current of the magnetron 60 is safely limited by virtue of the resistive conductor 39 connected in series with the input winding Tp of the transformer T. As a positive voltage pulse applied to the anode 60a initially rises, the magnetron is non-oscillatory and causes no appreciable current through the transformer T. Accordingly, there is substantially no voltage drop across the resistor 39, and the high voltage applied to the magnetron anode reaches a critical firing value early in the sinusoidally shaped rising voltage pulse. As soon as the magnetron fires, however, and thus tends to draw substantial current from the secondary winding Ts, corresponding current flows through the circuit for the primary winding Tp, thereby creating a substantial voltage drop across the resistor 39. Such voltage drop reduces the voltage efiectively applied to the primary winding Tp, and so correspondingly reduces the output voltage of the secondary winding Ts, thus reducing output voltage of the rectifier 75.

In this way, theanode voltage of the magnetron 60 is reduced as its cathode current increases by virtue of the resistor 39 connected in the low voltage side of the supply circuit, and excessive cathode currents are prevented by this voltage regulating action. Yet, the current which flows through the resistor 39 as an incident to this voltage regulation function creates a considerable amount of heat by PR dissipation in that resistor. Because the resistor 39 constitutes a part of the element 25 disposed inside the chamber 10, there is no need to provide special cooling fins or fans to dissipate that heat, and indeed such heat is distributed within the oven chamber 10 to assist in heating the object 11. Because the resistor 39 is connected in the primary circuit of the voltage step-up transformer T, it operates at a relatively low voltage level (e.g., 220 or 110 volts as compared to 1500 volts in the secondary circuit of the transformer). Thus, the resistor 39 need not be especially and expensively constructed for high voltage operation, and at the same time the danger of such high voltages, and the necessity for special high voltage insulators, in the oven structure are avoided.

From FIGS. and 6, it will now be seen that when microwave heating of the object 11 is desired, it is only necessary to actuate the control switch C, thereby closing the contacts C1, C2 and opening the contacts C3 (FIG. 5). The magnetron 60 is thus supplied with a high pulsating D.C. operating voltage between its anode and cathode, and produces repeated bursts of microwave energy. Such energy is coupled to the element 25 (as well as to the element 24) and radiated by the latter in the oven chamber where it is absorbed by' the object 11 to cause heating throughout the latter. At the same time, however, considerable energy in the form of heat is dissipated by current flow through the resistor 39 while the latter is performing its voltage-regulating function, but this heat is produced in the oven chamber by the element 25 while that element is simultaneously radiating microwave energy. Such 1 R dissipation in the resistance 39 is thus caused advantageously to contribute to the heating or cooking of the object 11. The thermostatic sensor 71, if employed, will open its associated contacts 71a when the temperature of the object 11 is above a predetermined value, thus stopping the generation and radiation of microwave energy and also stopping the associated resistance heating contributed by the element 25.

The single element 25, therefore, performs three distinct functions, viz. (a) it radiates microwave energy, (b) it serves to regulate the high voltage supply 61 so that the cathode current of the magnetron does not become excessive, and (c) it creates useful heat by resistance heating action inside the chamber 10. In addition to these functions, means are provided selectively to connect the element 25 to a conventional voltage source so that it may heat the oven by resistance heating action when not being used with the microwave generator. This is here accomplished by the contacts C2 and C3 of the control switch C. When the latter is set to the position shown by FIG. 6, the contacts C1 are open so that the magnetron is ineffective, as previously described. The contacts C2 are also open to disconnect the resistor 39 from the high voltage supply 61, and the contacts C3 are closed to connect the resistor 39 across the input terminals 55a, 55b of the selector switch S. Thus, if resistance heating of the oven chamber is now effected'by closure of the main switch 52 and assuming the thermostat contacts 51a to be closed),

the resistor 39 is energized by a direct connection across the 220 volt A.C. supply lines L1, L2. The element 25 under these conditions generates heat by resistance heating action, and thus by itself or in conjunction with the resistors 44 and 45 in the element 24 (depending upon the position of the selector knob Sk) warms the chamber 10 and any object 11 therein.

The apparatus here disclosed is highly flexible in its possible modes of operation, despite the fact that only two relatively simple and compact elements 24 and 25 are disposed Within the oven chamber. By way of summary,

the several possible modes of operation may be tabulated as follows:

Mode 1 Resistance heating.Switch C is left in its normal position (FIG. 6), switch 52 is closed, and selector knob Slc is set in the off position. The chamber 10 is now warmed by resistance heating of element 25 with its resistive conductor 39 connected across voltage source lines L1, L2.

Mode 2 Resistance heating.Same as mode 1, but knob Sk is set to its low position. The chamber 10 is heated at a low rate by resistance heating of resistors 44 and 45 in element 24 connected in series across voltage source lines L1, L2 and by resistor 39 connected across lines L1, L2.

Mode 3 Resistance heating.-Same as mode 2, except that knob Skis set to its medium position. The resistive conductor 45 is connected across lines L1, L2; and the resistive conductor 39 is connected across lines L1, L2, so that the chamber 10 is heated at a medium rate.

Mode 4 Resistance heating.Same as mode 3, except that the knob Sk is set to the high position, so that the conductrve resistors 39, 44, 45 are all connected individually across lines L1, L2 to provide a high heating rate.

Mode 5 Microwave heating.Switch C- is set to the actuated position (FIG. 5) and main switch 52 is opened. The power supply 61 and the magnetron 60 are effective, so that elements 24 and 25 radiate-microwave energy in the chamber 10. The resistive conductor 39 in element 25 regulates the magnetron operating voltage, and the heat generated therein as a result of such regulation contributes to warming of the object 11.

Modes 6, 7 and 8 Microwave plus resistance heating-Same as mode 5, except that main switch 52 is closed and knob Sk is set to the low, medium, or high positions, respectively, so that elements 24 and 25 both radiate microwave energy and simultaneously add heat to the chamber 10 by resistance heating action.

By virtue of the present invention it is possible to cook objects which contain no appreciable amounts of moisture by a resistance heating action produced by the same elements which serve an important function in microwave heating. This can be done either with or without simultaneous microwave heating of moisture-containing objects disposed in the oven chamber. Moreover, moisture-containing food can be internally heated and cooked by microwave action and simultaneously surface browned by resistance type heating or broiling action. And, whenever the microwave heating mode is to be used, heat generated as an incident to voltage regulation in the high voltage supply is advantageously utilized simultaneously to assist in warming any objects in the chamber.

I claim as my invention:

1. In an oven, the combination comprising means defining an oven chamber, a metal element disposed in said chamber, means for supplying microwave electrical energy to said element to cause such element to act as a driven antenna radiating microwave energy therefrom and int-o the chamber, and means for passing current other than at microwave frequencies through said element to generate heat by PR dissipation therein.

2. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a metal element disposed in said chamber and including a resistive conductor, means for supplying microwave electrical energy to said element to cause such element to act as a driven antenna radiating microwave energy therefrom and into the chamber, and means for passing current at a frequency lower than microwave frequencies through said resistive conductor to generate heat in the oven by PR dissipation in conductor.

3. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a rod-like element disposed in said chamber and including an outer metal sheath surrounding and insulated from a resistive conductor, a source of microwaveelectrical energy, means for coupling said source to said sheath to cause the latter to act as a driven antenna radiating microwave energy within said chamber, and means for passing current of a frequency substantially lower than the microwave frequency range through said resistive conductor to heat the oven by PR dissipation in the conductor. 4. In an oven, the combination comprising structural means defining an oven chamber adapted to receive an object to be heated, a metal element disposed in said chamber, a source of microwave electrical energy, means for coupling said source to said element so that the latter serves as a driven radiating antenna for distributing microwave energy in said chamber, said element including an electrical resistance, and means for supplying current through such resistance from a conventional electrical power source.

5. In an oven, the combination comprising structural means defining an oven chamber adapted to receive and contain an object to be heated, rod-like element disposed in said chamber and including a metal sheath surrounding but insulated from a resistive conductor, a source of microwave electrical energy, means for coupling said source to said element so that the latter serves as a driven radiating microwave antenna, and means adapting said resistive conductor to be connected to a conventional electrical power source so that resistance heating action is produced in such conductor.

6. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a metal element disposed in said chamber to act as a radiating antenna and including a resistive conductor, first selector means for supplying microwave electrical energy to said elements to drive the latter as an antenna so that the object is heated by microwave radiation from the element, and second selector means for connecting said resistive conductor to a conventional electrical power source so that said chamber and any object therein are heated by resistance heating action.

7. In an oven, the combination comprising structural means defining an oven chamber adapted to receive an object to be heated, a metal element disposed in said chamber to act as a radiating antenna and including a resistive conductor, first means for heating the object by microwave energy comprising a source of electrical micro- Wave energy and means coupling the same to drive said element so that it radiates microwave energy into the chamber, second means for heating the object comprising means for connecting said resistive conductor to a conventional electrical energy source, and selector means for (a) rendering said first means effective by itself, (b) rendering said second means effective by itself, or (c) rendering both said first and second means effective.

8. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a rod-like element disposed in said chamber and including an outer metal sheath surrounding and insulated from an inner resistive conductor, a magnetron microwave generator having an output coupler, means coupling said output coupler to said sheath for driving the latter as an antenna so that it radiates into said chamber microwave energy generated by said magnetron, and means adapted to selectively connect said resistive conductor to a conventional electrical power source to generate heat in said chamber by PR dissipation in said conductor.

9. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a resistor disposed in said chamber, a magnetron for generating microwave electrical energy, means for coupling the output of said magnetron into said chamber to produce microwave heating action in the object, a high voltage supply for applying an operating voltage to said magnetron, said high voltage supply including input terminals adapted to be connected to a conventional AC. power source and means including said resistor interposed between said input terminals and said magnetron for supplying the latter with a high operating voltage, whereby heat dissipated in said resistor contributes to the heating of the object.

10. In an oven, the combination comprising means defining an oven chamber, a heater-type resistor disposed in said chamber, a magnetron having an anode and a cathode for generating microwave electrical energy, a high voltage supply for said magnetron including input terminals adapted for connection to an AC. voltage source, a stepup transformer having -a primary and a secondary Winding, means connecting said resistor in series with said primary winding across said input terminals, a rectifier having its input connected to said secondary winding and its output connected to said anode and cathode, and means for coupling microwave energy from said magnetron into said chamber, whereby microwave heating occurs in the chamber and energy dissipated in said resistor as a result of current flow therethrough appears in the form of heat within the chamber.

11. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a resistor disposed in said chamber, a magnetron for generating microwave electrical energy, means for coupling the output of said magnetron into said chamber to produce microwave heating action in the object, a high voltage supply for applying an operating voltage to said magnetron, said high voltage supply including input terminals adapted to be connected to a conventional A.C. power source and means including said resistor interposed between said input terminals and said magnetron for supplying the latter with a high operating voltage, whereby heat dissipated in said resistor contributes to the heating of the object, and selector means for disconnecting said resistor from said voltage supply and connecting it instead to a conventional electrical power source.

12. In an oven, the combination comprising means defining an oven chamber, a metal element disposed in said chamber and including a resistive conductor, an electrical microwave generator, a high voltage supply for said generator adapted for connection to a conventional electrical energy source, said voltage supply including voltage regulating means constituted by said resistive conductor connected therein, and means coupling said microwave generator to said element so that the latter is driven as an antenna to radiate microwave energy in said chamber while heat dissipated in said resistive conductor as an incident to its voltage regulating function heats said chamber.

13. In an oven, the combination comprising means defining an oven chamber, a metal element disposed in said chamber and including a resistive conductor, a magnetron for generating microwave electrical energy, a high voltage supply for said magnetron including means adapted for connection to a conventional electrical voltage source and a resistor effectively connected between the source and the magnetron to transmit current to the latter and to decrease the magnetron supply voltage as the magnetron current increases, said resistor being constituted by said resistive conductor so that heat dissipated therein as an incident to operation of said high voltage supply heats said chamber, and means for coupling the output of said magnetron to drive said element as an antenna so that the latter radiates microwave energy in said chamber.

14. In an oven, the combination comprising means defining an oven chamber, a rod-like element disposed in said chamber and including a metal sheath surrounding and insulated from a resistive conductor, a magnetron having an anode and a cathode for generating microwave electrical energy, a high voltage power supply for applying a high D.C. operating voltage between the anode and cathode of said magnetron, said power supply including means effectively connecting said resistive conductor to carry current corresponding to the current flow between said anode and cathode for reducing the anode-cathode voltage in response to an increase in the anode-cathode current, and means for coupling the microwave output of said magnetron to said sheath so that the latter acts as a driven antenna radiating microwave energy in said chamber. 7

15. In an oven, the combination comprising means defining an oven chamber adapted to receive an object to be heated, a metal element disposed in said chamber and including a resistive conductor; a magnetron having an anode and a cathode for generating microwave energy; a power supply for applying a high D.C. operating voltage between said anode and cathode; said power supply comprising input terminals adapted for connection to an AC. voltage source, a step-up transformer having an input winding and an output winding, means connecting said input winding and said resistive conductor in series across said input terminals, and a rectifier having an input connected across said output winding and an output connected across said anode and cathode; and means coupling the microwave output of said magnetron to said element so that the latter acts as a driven antenna radiating microwave energy in said chamber while the FR dissipation in said resistive conductor heats said chamber.

16. In an oven, the combination comprising means defining an oven chamber, a metal element disposed in said chamber and including a resistive conductor, a source of microwave electric energy means for coupling the output of said microwave source to drive said element as a radiating antenna, a normally ineffective high voltage supply for said microwave source including means for coacting with a resistor connectable in circuit therewith to regulate the magnitude of the high volt-age while dissipating heat in the resistor, and selector means for optionally (a) connecting said resistive conductor to a conventional voltage source so as to cause the conductor to dissipate heat in said chamber or (b) rendering said high voltage supply effective and connecting said resistive conductor in circuit with said high voltage supply as the resistor referred to above, sothat said element both radiates microwave energy in said chamber and contributes heat in the chamber by the dissipation which occurs in said resistive conductor as an incident to operation of the high volt-age supply.

17. 'In an oven, the combination comprising means defining an oven chamber, a metal element disposed in said chamber, said metal element being shaped in the form of a radiating antenna and including as an integral part thereof a resistive conductor, a magnetron for generating microwave electrical energy when supplied with an operating voltage, means coupling the microwave output of said magnetron to said element to cause the latter to act as a driven antenna radiating microwave energy into said chamber, a high voltage supply for said magnetron including input terminals adapted to be connected to a conventional voltage source, first selector means for selectively connecting said resistive conductor to a conventional volt-' age source, and second selector means for selectively connecting said input terminals to a conventional voltage source and simultaneously connecting said resistive conductor in circuit with said high voltage power supply to perform a voltage dropping function, whereby operation of said first selector means causes said element tocreate 1 R heat in said chamber, and operation of said second selector means causes said element to radiate microwave energy in said chamber while simultaneously creating 1 R heat as an incident to its voltage dropping function.

References Cited by the Examiner UNITED STATES PATENTS Schall 21910.55

RICHARD M. WOOD, Primary Examiner.

L, H. BENDER, Assistant Examiner, 

1. IN AN OVEN, THE COMBINATION COMPRISING MEANS DEFINING AN OVEN CHAMBER, A METAL ELEMENT DISPOSED IN SAID CHAMBER, MEANS FOR SUPPLYING MICROWAVE ELECTRICAL ENERGY TO SAID ELEMENT TO CAUSE SUCH ELEMENT TO ACT AS A DRIVEN ANTENNA RADIATING MICROWAVE ENERGY THEREFROM AND INTO THE CHAMBER, AND MEANS FOR PASSING CURRENT OTHER THAN AT MICROWAVE FREQUENCIES THROUGH SAID ELEMENT TO GENERATE HEAT BY I2R DISSIPATION THEREIN. 